1) Field of the Invention
The present invention relates to a rubber composition adapted for various rubber articles, such as tire, vibration-damping rubber, fender and the like, and more particularly relates to a rubber composition adapted to be used as a tread rubber of pneumatic tire having excellent grip performance.
2) Description of Related Art
Recently, pneumatic tires having high driving performance have been increasingly demanded corresponding to the improvement of the performance of automobile and to the developments of paved roads and high speed road network. Particularly, grip performance is an important property demanded in pneumatic tire, and is generally estimated by the acceleration performance and brake performance. Accordingly, a car provided with a pneumatic tire having higher acceleration performance and brake performance can be driven more accurately and safely at high speed.
In the conventional method for producing a pneumatic tire having high grip performance, it is necessary to use a tread rubber composition having a large tan .delta., which is obtained by using a styrene-butadiene copolymer rubber having a high styrene content (that is, having a high glass transition temperature) or by using a styrene-butadiene copolymer rubber blended with large amounts of process oil and carbon black.
As a method for preventing the lowering of grip performance of a tire due to the temperature rising, Japanese Patent Laid-open Application No. 187,011/84 discloses the use of a copolymer rubber obtained by copolymerizing a monomer, such as 1,3-butadiene, styrene, isoprene or the like, with an acrylate or methacrylate compound containing diphenyl phosphate group, such as diphenyl-2-methacryloyloxyethyl phosphate, diphenyl-2-acryloyloxyethyl phosphate or the like.
However, when a styrene-butadiene copolymer rubber having a high styrene content is used in order to obtain a tire having high grip performance, the resulting tire has surely high grip performance, but the tan .delta. value of the rubber is lowered due to the temperature rising of the rubber caused by the running of the tire, and the grip performance of the tire is noticeably lowered. While, although a tire having high grip performance can be obtained by using a styrene-butadiene copolymer rubber composition having high contents of process oil and carbon black, there are certain upper limit values in the contents of process oil and carbon black in the rubber composition, and a rubber composition containing extremely large amounts of process oil and carbon black is very poor in the breakage resistance and wear resistance. Further, the method described in the above described Japanese Patent Laid-open Application No. 187,011/84, which aims to prevent the lowering of grip performance of a tire due to the temperature rising of rubber constituting the tire, has such a drawback that the method can not be applied to natural rubber and further damages excellent properties inherent to polymers, for example, styrene-butadiene copolymer rubber, polybutadiene rubber and the like, depending upon the production condition of the polymers.